This invention relates to a sensing device and, more particularly, to an optical displacement sensing device that utilizes the interference of light beams for detecting relative displacements of a diffraction grating with reduced sensitivity to misalignments.
An optical displacement sensing device, or optical encoder, is described that can overcome several of the most significant problems faced by designers of these types of devices for practical high precision measurement. If an optical displacement sensing device is to be used to make high precision measurements of a grating surface displacement, e.g., sub-micron resolution and accuracy, it must effectively eliminate or attenuate any distortions of such measurements at a very high level. Current optical displacement sensing devices are incapable of economically and practically eliminating or significantly reducing the effects of certain distortions or parameter drifts from their measurements at the desired levels of accuracy and resolution. Among the problems frequently encountered while using such devices are those arising from a change in wavelength of the light source used for performing measurements, such as associated changes in diffraction angles, changed lights paths, and changes in the number of wavelengths occurring in two light paths of differing length, which affects their relative phase and interference pattern.
Another problem is associated with very small grating periods. To achieve high measurement resolution, it is desirable to use a scale grating with as short a grating period, d, as possible. The lower limit is set by the wavelength xcex of the light source, according to the formula d greater than xcex/2. However, unless special design measures are taken, an encoder using such a short scale grating period is too difficult to align with sufficient yaw accuracy, requiring expensive equipment or excessive time and care during installation. Yaw misalignment is a rotation of the optical readhead relative to the grating, in a plane parallel to the grating.
With a yaw misalignment, the desired output beams arising from the scale are no longer parallel and related xe2x80x9cdistortionxe2x80x9d interference fringes are created. If the distortion interference fringe period is small relative to the diameter of the detector area illuminated by the interfering beams, the desired modulation of the signal due to grating motion will be significantly attenuated because several distortion fringe periods will fit within the detector area, and the detector signal will tend toward the constant average intensity of these distortion fringes. To avoid this effect when using practical types of optical detectors and a grating period that approaches the previously discussed xcex/2 limit, the yaw misalignment must be smaller than approximately 0.1 milliradians, in the absence of special design measures. Such alignment requirements are impractical for many users and applications. It is known to incorporate retroreflectors in the light paths of optical encoders in order to overcome such yaw problems, as shown in U.S. Pat. Nos. 5,079,418 to Dieter and 4,930,895 to Nishimura, each of which is incorporated herein by reference, in its entirety. However, such arrangements of retroreflectors have not simultaneously considered compact and economical optical readhead design and packaging, the versatility to work with grating periods both greater and lesser than the wavelength of the light source, and measurement insensitivity to various parameter driftsxe2x80x94including misalignments other than yaw. All of these design factors must be considered simultaneously, and the proper tradeoffs chosen, in order to robustly achieve currently desired measurement resolutions and accuracies.
In particular, parameter drifts including dynamic position misalignments are important error sources in an optical displacement sensing device. Herein, the term dynamic misalignment or drift means the change in an alignment component or parameter that occurs between one displacement position and another displacement position, or at the same displacement position over a period of time, for any reason. Among the range of possible dynamic position misalignments and drift that are introduced in practical applications are changes in the gap between the readhead and grating, pitch (rotation about an axis parallel to the grating and normal to the measuring axis), yaw, roll (rotation about an axis parallel to the measuring axis), and drift in the wavelength of a light source. U.S. Pat. No. 5,146,085 to Ishizuka, incorporated herein by reference, in its entirety, and the ""895 patent, previously incorporated, both disclose optical readhead configurations which are relatively insensitive to errors associated with pitch. However, these configurations are not versatile and robust enough when considering the placement of retroreflectors in combination with consideration of the other design factors noted for simultaneous consideration above. Thus error sensitivities associated with the relative pitch of the readhead and grating remain as some of the most difficult error sources to reduce in practical high resolution encoder design and application.
Furthermore, the ""085 and ""895 configurations may introduce problems created by the reflection of a light beam into the light source used for the generation of the light beam, leading to instability in the wavelength of the light source. Also, these configurations require polarizers, which attenuate the light available to the optical detector and which may limit or prohibit the configuration for use with certain detectors and/or impose relatively higher system power requirements, which may complicate or limit their use in certain applications.
The present invention is directed to optical readhead configurations which are suitable for compact and economical design and packaging, versatile enough to apply with grating periods both greater and lesser than the wavelength of the light source, and which are substantially simultaneously insensitive to various parameter drifts including at least dynamic yaw and pitch misalignments. Some configurations also avoid or limit attenuation of the light available to the detector and/or avoid reflection of light beams into the light source.
In accordance with this invention, an optical displacement sensing device or optical encoder readhead is provided for determining the relative displacement of a diffraction grating scale along a measuring axis. The grating may be reflective and the grating pitch may be less than the wavelength of the light of the encoder readhead. In one embodiment, the sensing device includes a split light beam input portion for inputting two split light beams along respective light paths; light beam directing elements for directing the two split beams along respective converging light paths toward a first zone on the scale grating to give rise to two diffracted beams along respective light paths which diverge to enter two retroreflector elements that receive the two diffracted beams and retroreflect them as two respective retroreflected beams along converging light paths toward a second zone on the scale grating to give rise to two later-diffracted light beams. The two later-diffracted light beams are then directed to a shared zone, and an optical detector. The optical detector detects at least one illumination characteristic arising from the shared zone, thus sensing displacement of the grating scale along a measuring axis.
In accordance with one aspect of the invention, by directing the two split beams along respective converging light paths toward a compact first zone on the scale grating, the device sensitivity to dynamic pitch misalignments can be reduced.
In accordance with another aspect of the invention, by retroreflecting the two diffracted beams along respective converging light paths toward a compact second zone on the scale grating, yaw sensitivity is substantially eliminated while the device sensitivity to dynamic pitch misalignments is further reduced.
In accordance with another aspect of the invention, the sensing device configuration may be aligned with two different inclined planes inclined away from each and parallel to the measuring axis. Various inclined configurations can prevent undesirable mixing of various light beams, while at the same time facilitating compact design and packaging of the device.
In accordance with a further aspect of the invention, in one embodiment the two split beams converge along respective light paths until reaching the first zone on a scale grating, and the retroreflectors are located and oriented relative to their received beams such that their two retroreflected beams converge back along respective converging optical paths until reaching a hypothetical plane parallel and proximate to the second zone on the scale grating and then diverge over a small remaining distance to the second zone on the scale grating. This arrangement produces a configuration of path lengths and light spots on the scale grating that further reduces the device sensitivity to dynamic pitch misalignments, while at the same time facilitating compact design and packaging of the device.
In accordance with a further aspect of the invention, in a separate embodiment the two split beams converge along respective light paths until reaching a hypothetical plane parallel and proximate to the first zone on the scale grating and then diverge over a small remaining distance to the first zone on the scale grating, and the retroreflectors are located and oriented relative to their received beams such that their two retroreflected beams converge back along respective converging optical paths until reaching the second zone on a scale grating. This arrangement produces a configuration of path lengths and light spots on the scale grating that further reduces the device sensitivity to dynamic pitch misalignments, while at the same time facilitating compact design and packaging of the device.
In accordance with another aspect of the invention, the respective light paths which diverge to enter the two retroreflector elements each receive a diffracted light beam arising from only one of the split beams, avoiding the need for polarizers in the encoder readhead.
In accordance with a further aspect of the invention, the light paths of the two later-diffracted light beams each receive a later-diffracted light beam arising from only one of the retroreflected beams, further avoiding the need for polarizers in the encoder readhead.
In accordance with another aspect of the invention, in various embodiments, the light beams that are directed toward the first zone impinge upon the scale grating at a first respective pair of light spots that are separated by a first distance along the measuring axis direction, and the respective light beams that are directed toward the second zone impinge upon the scale grating at a second respective pair of light spots that are separated by a second distance along the measuring axis direction. In one embodiment, the first and second distances are made to be equal. In a further embodiment, the first respective pair of light spots and the second respective pair of light spots are nominally aligned along a direction perpendicular to the measuring axis.
In accordance with another aspect of the invention, the sensing device configuration may be arranged such that the split light beams nominally impinge on the first zone within a range of angles that attenuates the sensitivity of the device to certain errors associated with at least dynamic pitch changes and wavelength drift, while at the same time facilitating compact design and packaging of the device.
In accordance with another aspect of the invention, the light beam directing elements comprise opposing surfaces of a block of transparent material, providing a compact, economical and robust way to fabricate and precisely position the elements.
In accordance with another aspect of the invention, the retroreflector elements comprise respective corners of a block of transparent material, providing a compact, economical and robust way to fabricate and precisely position the retroreflector elements.
In accordance with another aspect of the invention, a readhead grating produces the two split beams and the sensing device configuration may be arranged such that the readhead grating and retroreflectors are in a configuration that attenuates the sensitivity of the device to certain errors associated with at least dynamic pitch changes and wavelength drift.
In accordance with a further aspect of the invention, the sensing device configuration includes an input grating at a first height and an output grating at a second height, wherein the retroreflectors are located at a height that is midway between the height of the input grating and the height of the output grating.
In accordance with another aspect of the invention, a beamsplitter produces the two split beams and the sensing device configuration may be arranged such that the beamsplitter and retroreflectors are in a configuration that attenuates the sensitivity of the device to certain errors associated with at least dynamic pitch changes and wavelength drift.
In accordance with a further aspect of the invention, in various embodiments a polarizing configuration may be used for the optical detector and in various other embodiments a compact optical array may be used for the optical detector.
Hence the invention overcomes the disadvantages of prior art optical displacement sensing devices with a compact, economical and versatile configuration applicable to grating periods both greater and lesser than the wavelength of the light used, and which is substantially simultaneously insensitive to various parameter drifts including at least dynamic yaw and pitch misalignments.